“We’re going to continue to operate in Lawrence,” vowed Aaron Feuerstein. ‘‘We had the opportunity to run to the South many years ago. We didn’t do it then, and we’re not going to do it now.”

Feuerstein’s promise to rebuild here was desperately needed reassurance for an economically battered area reeling from a Monday night explosion and fire at the textile complex that threw 1,400 employees out of work and injured about 36 persons, eight critically. Among the injured were a half-dozen firefighters, none seriously.

The cause of the fire was still unknown yesterday as investigators combed through the smoking rubble of three devastated factory buildings. Speculation on a cause centered on the possibility that a boiler exploded with such force that it knocked out a new, modern sprinkler system in the mill complex.

Malden Mills owner Aaron Feurerstein paused during an interview. He vowed to rebuild the facility.

Fire and Malden Mills officials said they put a dollar estimate on the damage caused by the fire, but company officials said the firm was insured for its loss.

Although officials declared the fire under control yesterday, one area burst into flames just before nightfall, and the fire was not expected to be fully extinguished until today.

Meanwhile, 61 residents who had been evacuated Monday night began returning to their homes yesterday after tests found that no harmful level of toxic chemicals had been released into the air by the explosion. One nearby house was damaged and a business destroyed by the fire.

Gov. William F. Weld, at a news conference here yesterday, declared a state of emergency in Methuen and Lawrence, which will accelerate the process of delivering disaster aid to the area. Weld also promised to file emergency legislation to ensure that those left jobless by the fire will receive unemployment checks before Christmas.

And he pledged state aid for Malden Mills, a firm that pays relatively high wages to recent immigrants and longtime Lawrence-area residents alike. “If there’s one employer we are not going to let fail, it’s this one,” said Weld.

Meanwhile, Sens. Edward M. Kennedy and John F. Kerry, who met with Malden

Mills workers yesterday in Methuen, later extracted pledges in Washington from Secretary of Labor Robert Reich and other federal officials to help Malden Mills workers.

Reich said he would deliver the first $1 million in emergency assistance today. In addition, Reich said he will process the state application for $3.7 million in emergency assistance within 24 hours.

Reich’s pledge came after Kennedy, Kerry and Rep. Martin Meehan, a Lowell Democrat, convened a meeting of federal officials at Kerry’s office yesterday afternoon. “I am convinced we will have an unprecedented response,” said Kerry.

Reich said the federal government would provide unemployment compensation benefits beyond the usual 48 weeks of coverage to those thrown out of work by the fire. Because it will take months for Malden Mills to rebuild the plant and replicate the unique manufacturing equipment used in the creation of its fabric, such a benefit extension was viewed as particularly important by members of the state’s congressional delegation.

Feuerstein, the mill owner, said yesterday that he does not understand ‘‘how the fire moved so quickly, because we have installed the most modern sprinkler system.”

“But it happened,” he added. “The only thing I can do is resurrect a horrible situation. This is no time for me to feel sorry for myself.”

Walter Bickford — the company’s director of environmental affairs, health and safety — said the firm lost a third of its manufacturing capacity. However, he said that within a month the company could resume production of 400,000 yards of cloth. Meanwhile, although 1,400 factory workers are out of work, another 900 office workers will remain on the job.

Records from the Occupational Safety and Health Administration show that Malden Mills was inspected 13 times since 1980. Five investigations resulted in more than $38,000 in fines. The most recent fine stemmed from an explosion in March 1993 that injured four workers.

Feuerstein acknowledged there had been fines by OSHA but insisted that the safety issues cited were “minor, minor, minor.” Several union leaders yesterday praised Feuerstein, saying he is committed to upgrading the safety of the workplace.

Even a day later, the speed and ferocity of Monday night’s blaze still had some officials shaking their heads. Bourassa, the fire chief, recalled watching as flames jumped from the fourth floor of one building to the fourth floor of another.

“This was a hard fire to fight,” Bourassa said. “The guys did everything right. They worked to contain the fire. They worked to get in front of the fire, and still the fire got away from them.”

Lawrence firefighter William Pierce was among those injured in the fire. He was treated at Hale Hospital in Methuen and released.

Lawrence Mayor Mary Claire Kennedy said several local mill owners with vacant space have called her to offer the space to Malden Mills, in some cases rent-free for six months.